Astroparticle Physics

56 4 Physics of Particle and Radiation DetectionFig. 4.5Production of a Cherenkov ring ina water Cherenkov counterpoint back to their sources, while hadrons only produce anisotropic background. The axis of the Cherenkov cone followsthe direction of incidence of the primary photon. TheCherenkov cone for γ -induced cascades in air spans only±1.4 ◦ , therefore the hadronic background in such a smallangular range is relatively small.Apart from the atmospheric Cherenkov technique, theCherenkov effect is also utilized in large water Cherenkovdetectors for neutrino astronomy. The operation principle ofa water Cherenkov counter is sketched in Fig. 4.5. Cherenkovradiation is emitted along a distance x. The Cherenkovcone projects an image on the detector surface, which is ata distance d from the source. The image is a ring with anaverage radiusr = d tan θ C . (4.12)4.4 Special Aspects of Photon Detection“Are not the rays of light verysmall bodies emitted from shiningsubstances?”Sir Isaac NewtonZof the absorber material100806040photoelectriceffectdominantpairproductiondominant20 Coulomb scatteringdominant010 –2 10 –11 10 10 2photon energy [MeV]Fig. 4.6Domains, in which various photoninteractions dominate, shown intheir dependence on the photonenergy and the nuclear charge ofthe absorberThe detection of photons is more indirect compared tocharged particles. Photons first have to create charged particlesin an interaction process. These charged particles willthen be detected via the processes described, such as ionization,excitation, bremsstrahlung, and the production of Cherenkovradiation.At comparatively low energies, as in X-ray astronomy,photons can be imaged by reflections at grazing incidence.Photons are detected in the focal plane of an X-ray telescopevia the photoelectric effect. Semiconductor counters, X-rayCCDs 2 , or multiwire proportional chambers filled with anoble gas of high atomic number (e.g., krypton, xenon) canbe used for focal detectors. These types of detectors providespatial details, as well as energy information.The Compton effect dominates for photons at MeV energies(see Fig. 4.6). In Compton scattering, a photon of energyE γ transfers part of its energy E to a target electron,thereby being redshifted. Based on the reaction kinematics,2 CCD – Charge-Coupled Device (solid-state ionization chamber)